Transgenic C57Bl/6 mice (Big Blue tm) carrying the neutral reporter gene ZAP/lacIq can be rescued using a lambda shuttle vector and the mutant frequency determined after treatment with potential environmental chemical mutagens. We have compared single versus multiple dosage regimens and determined the induced mutant frequencies for the carcinogen, ethylnitrosourea, and 1,2,3- trichloropropane, which induced liver tumors in B6C3F1 mice in a two year carcinogenesis study. Using an optimized lambda shuttle vector rescue protocol, we have made several important observations concerning this in vivo mutagenesis assay system: (1) EtNU induced the same mutant frequency in both the left lateral or medial lobe of the liver of transgenic mice in independent assays and these were significantly increased above control mutant frequencies. Interlaboratory comparisons using aliquots of DNA from transgenic mice treated with 1,2,3-trichloropropane and the same optimized protocol compare favorably. Together these results suggest that the assay has been optimized to reduce variability and that identification of potential environmental mutagens from different laboratories are possible. (2 )Mutant frequencies observed in the liver of these mice are dependent upon mutagenic potency, dose, dose rate, expression time, and the quality of the overall assay. Less potent mutagens, e.g. 1,2,3- trichloropropane, require more extensive packaging and plaque forming unit observations in order to approach the number of observed mutant to determine statistical significance. This transgenic mouse model (Big Blue tm; ZAP/lacIq) offers important advantages to investigate tissue specific in vivo mutagenesis.